Add Vitest test setup and CircleCI CI pipeline

[circleci-app]

Summary

• Vitest installed as a dev dependency (natural fit since the project already uses Vite — shared config, fast HMR-style reloads in watch mode)
• vitest.config.ts created with the @common path alias so tests can import shared types cleanly
• npm test script added (vitest run for CI; vitest alone gives watch mode locally)
• Initial unit tests in src/common/common.test.ts covering the pure URL-classifier utilities (isChromeURL, isFirefoxURL) — 9 tests, all green
• .circleci/config.yml created with:
  • Node 22 Docker executor (matches the project's runtime)
  • ~/.npm cache keyed on package-lock.json checksum to speed up repeated runs
  • Three sequential steps: npm run lint → npm test → npm run build

Test plan

• Confirm CircleCI pipeline triggers on push and all three steps pass
• Run npm test locally to see the 9 passing tests
• Add more tests alongside new utility functions as the codebase grows

https://app.circleci.com/agents/circleci/977715b4-c847-474d-bc08-01d21c55ef7e/chat/f7f0958f-1476-4196-8398-6e6e90c991bc